It is known that one of the main problems encountered in advancing a flexible element inside a very long cavity consists of the poor ability of the flexible element to absorb resistance to advancement without distortion in the part not yet inserted in the cavity. Said undesirable distortion of the flexible element, in addition to being a factor of rapid wear of said element, makes the advancing motion of the flexible element irregular with resulting reduction of the quality of the work performed and increase of costs due to a greater number of passes to obtain the same results than there would be if motion were controlled in a uniform manner. Finally where the flexible element is a hose containing a liquid under very high pressure, e.g. up to 3,000 bar, as in the case of cleaning the internal surfaces of the tubes of a tube nest, irregular advancement of the hose can be very dangerous for the operator who has to handle said hose. The operations which require the use of a hose to be inserted in a very long longitudinal cavity are normally performed in a nonautomatic manner and manually. The operator inserts the end of the hose into the longitudinal cavity in which said hose must run and pushes the hose until it reaches the other end of the cavity. If the flexible element must remain in the same place, (e.g. if it is a cable), the operation is terminated. If the hose was used for inspection or cleaning the motion is reversed and the hose is drawn back until it goes out of the end of the cavity in which it had been inserted.
In the case of cleaning or internal treatment of tube bundles with pressurized liquids, the insertion and withdrawal of the hose are performed tube by tube with a single hose and interrupting the flow of liquid which passes through the hose before withdrawing it from each tube and turning on flow again after insertion in the next tube.
It is just these insertion and withdrawal operations which represent the greatest danger for safety of the operator in case he does not turn off in a timely manner the liquid in the hose before its withdrawal from each tube. The liquid jet under very high pressure which comes out of the operating end of the hose, no longer contained by the walls of the tube being treated, can very seriously injure the operator or persons in the vicinity of the tube nest being treated.
It is thus clear why tube nests are not cleaned directly on the plant where they are installed but are disassembled and placed on supports which make their treatment easier and safer. A second defect of these manual operations consists of the fact that the operator tends to stop the flow of liquid inside the hose well before withdrawing it from the tube being worked on to increase his own safety and this circumstance reduces the effectiveness of the cleaning because a considerable part of the tube undergoes a single cleaning pass and not both passes of the advancing and withdrawal phases.